If you want a system with two 7950 GX2s (I know I'm getting flamed for this), just buy a computer from Falcon Northwest or VoodooPC. They'll be somewhere around 10,000 apiece, so they're not for the faint of wallet.
Just out of curiosity, at what resolution do you game? You'll see the biggest benefits of SLI/Quad-SLI at resolutions of 1600 x 1200 or higher, so the only reason you'll need a system with that much graphics push is if you're going to buy one or two of those 30" Apple Cinema or Dell displays.